The present invention relates to flywheel energy conversion devices that include motor-generators and methods for heating rotors using the circuitry of the flywheel energy conversion devices.
Motor-generator systems typically use some type of rotor to incite motoring or energy generating action. For example, a rotor may be used in an automobile alternator to provide electrical power to the car""s electrical system. Other applications may include using the rotor in a motoring function to drive, for example, a power drill. An example of a large scale application may involve rotating a rotor with a prime mover such as steam-driven turbine of a nuclear facility to generate electricity for a utility power grid. Yet another example may include rotors that are used in flywheel energy conversion devices such as that described in Clifton et al. U.S. Pat. 5,969,457 (""457 patent), which is hereby incorporated by reference in its entirety.
Each of the rotors described above may exhibit material properties that require specific operating temperatures to operate safely. For example, rotors in flywheel devices may require an operating temperature of at least 15xc2x0 C. before normal operation can commence. Therefore, to ensure safe operation of the rotor, the temperature of the rotor should be greater than a specified temperature to provide a substantial margin of safety against brittle fractures. (The specified temperature may be dictated by the material properties of the rotor itself, in that the impact energy of metallic materials is a non-linear function of temperature. It is also known that some material strength properties (e.g., impact energy and fracture toughness) are hampered when the temperature of the material is low. But the same material may exhibit resilient strength properties at higher temperatures.) If a rotor is forced to spin at a relatively high speed when its temperature is below a specified value, the stresses due to rotation may cause the rotor to shatter, crack, or experience brittle fracture.
Various preheating techniques have been applied to motor-generator applications to ensure that the rotor operates in a preferred operating temperature regime. For example, the process of preheating a rotor of a 500,000 VA generator is described as follows. In this example, steam may be used to transfer heat to the rotor and other associated components (e.g., turbine disks) by convective means. Initially, a limited quantity of steam may be introduced to gradually warm the rotor. Then, in controlled increments, greater quantities of steam may be applied to steadily raise the temperature of the rotor to a desired level. The heating process may progress over a period of several hours to several days, but is necessary to prevent potential damage that can be caused by changing temperature gradients that can inflict thermal stresses on the rotor. This heating process may be problematic because it is cumbersome, time consuming, and requires an external source of heat (e.g., steam) to raise the rotor temperature. Once the heating process is complete, however, the rotor is in condition to safely generate power.
Other known preheating methods have been used for preheating devices such as engines. For example, preheating an engine may involve installing heater coils strategically around vital parts of the engine. Power may be provided to heater coils so that they radiate heat to the engine via convective or conductive means. Eventually, this radiated heat may preheat the engine to a specified temperature prior to ignition. But this method requires the addition of external components (i.e., heater coils) to enable the preheating process.
In view of the foregoing, it is an object of this invention to preheat the rotor of a flywheel energy device prior to normal use.
It is a further object of this invention to preheat the rotor to obtain a substantially high margin of safety against brittle fracture or other damage.
It is also an object of this invention to use the circuitry of the flywheel energy device to preheat the rotor prior to normal use.
These and other objects of the present invention are accomplished in accordance with the principles of the invention by preheating the rotor of a flywheel energy conversion device prior to use. In preferred embodiments, the rotor may be preheated using the circuitry provided with the flywheel device. Such circuitry may include armature windings, field coil windings, electronics (including software), temperature sensors and other suitable features of the flywheel device.
The rotor is preferably preheated prior to use to provide a reasonable margin of safety that protects the rotor from incurring brittle fractures. Since brittle fractures are more likely to occur at lower temperatures, the present invention may implement several techniques to raise the rotor temperature to a safe operating level. These problems are particularly relevant when the temperature is below a specific transition temperature, which is dictated by the material properties of the rotor. This may be advantageous because it enables a flywheel device to operate in sub-zero arctic temperatures.
The present invention may utilize convective and radiative methods to heat the rotor. Convective methods are particularly useful if the rotor is not operating in vacuum conditions. An example of using this method may involve heating the field coils by passing current through them. As the current flows through the coils, they generate heat, and that heat may be transferred to the rotor via air, which serves as a heat transfer medium. After time, the convected heat may eventually raise the temperature of the rotor to the desired level.
In another embodiment, a method of radiative heating may be used. This technique utilizes high frequency magnetic induction to heat to the rotor. This may be advantageous because heat can be conveyed directly to the rotor without requiring any physical application of a device (e.g., heat iron) to the rotor. A further advantage of this technique is that the circuitry of the flywheel device can be used to induce the heat energy to the rotor. This eliminates the need for additional equipment to perform the heating process. In addition, heat may be imparted onto a rotor operating in a vacuum, an environment where convective methods cannot be employed.
Using the circuitry of the flywheel device, induction heating may be provided as follows: 1) provide a high frequency current (produced by flywheel device electronics) to the armature windings; 2) use the high frequency current to generates flux, which passes through the rotor; 3) induce currents in portions of the rotor where the flux passes through; and 4) generate heat from the induced current to raise the rotor temperature.
The induction heating process may be a single process or it may be separated into a preheating step and a settling step. During the preheating step, high frequency currents are continuously applied to the armature windings. It is during this step in which certain portions (e.g., toothed protrusions) of the rotor are constantly subjected to induced currents. Since these portions are constantly receiving induced currents, they may be heated to relatively high temperatures, whereas the rotor core, which does not have induced currents, may remain relatively cool.
Since heat may continue to flow from the inductively heated portions even after cessation of high frequency currents, it may be useful to transition from the preheating step to the settling step to avoid potentially overheating the flywheel device (e.g., the armature windings or rotor surface). The settling step may provide time for surface heat of the rotor to diffuse and fully penetrate the rotor such that the desired temperature is provided substantially throughout the rotor.
The present invention may also use different techniques to determine whether the rotor requires heating prior to use. Both indirect and direct rotor temperature measurements may be performed (either individually or in combination with each other). The indirect method may include monitoring the temperature of the stator (e.g., armature winding), outer casing, or some other portion of the flywheel device to obtain an approximate temperature of the rotor. If the monitored temperature is too low, then the electronics may determine how long to heat the rotor to obtain a desired temperature. If the induction heating technique is used, then the electronics may determine the preheating and settling time needed to safely heat the rotor.
If direct rotor temperature measurement is implemented, a device such as an infrared detector may be positioned within the flywheel to monitor rotor temperatures directly. The infrared detector may provide substantially accurate rotor temperatures, which may enable the electronics to more optimally heat the rotor by minimizing heating times and reducing risk of overheating the flywheel device.